On Black Friday, a B+ (for Bargains); At High and Low End, Season Starts With Quest for Good Price
Shoppers from all over country pile into stores on first official day of holiday season--known to retailers as Black Friday, because they hope it will push their bottom lines into black; while retailers tend to monitor activity obsessively over Thanksgiving weekend, kickoff to crucial holiday buying period has lessened in importance in recent years as more and more shoppers choose to buy closer to Christmas; Thanksgiving weekend can also be deceiving; last year, season started with bang and ended on mediocre note; graph; photo (M)